An “evacuated space” basically may be any hermetically-sealed off space in which a more or less high vacuum is to be maintained. Such an evacuated space, referred to below as “vacuum space”, may, for example, be part of a container or of a pipe, or a pipeline for carrying a cooled medium, for example, a low-temperature liquid gas, around which is mounted a vacuum insulation closed off outwardly by means of a metallic tube and operated under a vacuum. The vacuum space with vacuum insulation may also be the interspace between two concentric metal tubes of a cryostat for superconductive cables. Further statements relate to vacuum insulation of this type as representing all other possibilities of use.
In order to detect leakage, referred to below as, in brief, a “leak”, in the “envelope” of a vacuum space, a response threshold of a few kPa is sufficient. Pressures in the region of 10−3 Pa are customary for vacuum insulation. The vacuum insulation largely loses its effectiveness in the region of 0.1 Pa. If a leak occurs in the outer tube delimiting the vacuum insulation, a pressure of approximately 105 Pa is reached after a short time, whereas in the case of a leak in the inner tube, a pressure of, for example, up to 2 MPa, corresponding to the operating pressure, may occur after a short time.
To monitor the pressure in a vacuum space, it is known to use a friction vacuum gauge which has a ball which is located in a small tube connected to the vacuum space and which is brought into suspension and set in rotation from outside by means of external magnetic fields. The friction of this ball with respect to the surrounding gas is dependent on the pressure of the latter. It can be determined by means of corresponding evaluation electronics as a measure of the pressure or of the residual pressure in the vacuum space. The outlay is very high on account of the measuring instruments required.
The known arrangement according to DE 101 117 329 A1 mentioned in the introduction is a pipe-break protection for a vacuum-insulated filling line. Connected to the filling line is a bellows which is closed off by means of a contact plate and around which is arranged a guide tube which is axially shorter than the bellows in the expanded initial position. In the operating position, the contact plate of the then shortened bellows bears against the end face of the guide tube. The contact plate is connected to a pneumatic or electrical contact maker. In the event of a pressure loss in the vacuum insulation of the filling line, the bellows expands. At the same time it takes up the contact plate which is thereby moved away from the contact maker, so that the latter is no longer activated. The supply of a cryogenic medium to the filling line is then interrupted. This known arrangement can be employed only restrictedly or even not at all for use outside buildings, because the bellows is not protected against environmental influences and may easily ice up.